Apparatus for producing and shaping a beam of charged particles



Aprzi 3, 1962 F. SCHLEiCH 3,023,491

APPARATUS FOR PRODUCING AND SHAPING A BEAM OF CHARGED PARTICLES I FiledJune 22. 1959 Patented Apr. 3, 1962 APPARATUS FOR PRODUCING AND SHAPINGA BEAM OF CHARGED PARTICLES i Fritz Schleich, Unterirochen, Aalen,Germany, assignor to Carl Zeiss, Wurttemberg, Germany Filed June 22,1959, Ser. No. 822,000 Claims priority, application Germany June 20,1958 3, Claims. (Cl. 250-495) This invention relates to the productionof beams of charged particles and, more particularly, relates toapparatus for producing and shaping beams of high energy chargedparticles into an impinging beam of high intensity across a sharplydefined cross section.

Apparatus utilizing a beam of charged particles focussed upon a materialto be worked thereby are known to the art. In such apparatus, thefocussing lens has been an axially symmetrical lens. The lensaberrations and, particularly the spherical aberration of an axiallysymmetrical lens, makes it very difiicult to focus a beam having a largefocal aperture. When focussing such a beam with an axia y symmetricallens one obtains an -irnpinging beam which has a nonuniform intensitydis- 'tribution throughout the beam cross section. The ina WW than 't ein ensity in e marginal zones. For this reason oerthe central area orHie impinging beam can be used for working the material.

In order to work material by a beam of charged particlesit is desirable,however, to use a beam which has a large focal aperture. Such a beam hasa great intensity and allows to obtain very good working efiectsespecially in drilling, milling or welding materials. The speed ofworking and the precision of control is afforded by having the intensityin the impinging beam high and by having the beam peripheral edgessharply defined by a rapid decrease of intensity.

It is, therefore, the object of this invention to provide an apparatusfor producing and focussing a beam of charged particles having a largefocal aperture which will have a uniformly high intensity throughout thebeam cross section taken at the work piece and which will have a sharplydefined peripheral edge at which the beam intensity will decreasesharply.

In accordance with this object there is provided, in accordance with apreferred embodiment of this invention, a cathode, an associated controlgrid and an anode for producing a beam of charged particles having alarge focal aperture. Such apparatus will produce high beam energy withtolerable cathode emission current density levels. A cylindrical lens ispositioned to focus the beam upon the surface of material to be workedthereby. The cylindrical lens is corrected for aberrations and has a.large angular field of view which can encompass the relatively largeemitting surface of the cathode of the beam producing system. Thefocussing of the beam en-. ergy distributed over. a large angular fieldby an aberration-corrected cylindrical lens provides a beam impingingupon the material having a substantially rectangular plot of energyintensity across the cross section.

It is often advantageous to employ a linearly developed cathode, thatis, a cathode having a dimension along one axis greater than thedimension along a co-ordinate axis. With such a cathode, it is possibleto utilize a larger emitting cathode surface than is the case with roundcathodes. In such embodiments it is desirable to align the direction ofthe linearly extending cathode with the main axis of the cylindricallens serving to focus the beam. In such arrangement, a line focus willbe formed on the material instead of the round focal point usuallyassociated with beam working apparatus. A line focus is not directlyuseable in certain applications as: for example, the drilling of roundholes in the material to be worked. In such applications it is desirableto employ a rotating field between the material and the lens to rotatethe beam about its center of area to give a working beam of eriectiveiycircular cross section.

The cylindrical lens is preferably an electromagnetic cylindrical lens.Such a lens has the advantage of permitting ease of variation in thelength of beam focal line by merely changing the coil current. Thus,when employed with a rotating field for drilling round holes in amaterial, the diameter of the bore can be varied by variation of fieldcurrent in the magnetic lens. Thus, control overthe diameter of the boreover a predetermined range is afforded.

In those applications where it is desired to employ the focussed beamfor the purpose of producing profiled borings or milling of the materialsurface, the use of a cylindrical lens in combination with a rotatingmagnetic field requires relatively complex beam control apparatus. Insuch cases it is desirable to use two or more cylindrical lenses spacedapart along the axis of the beam. The axis of the cylindrical lenses arearranged at right angles, one to the other. Such arrangement will allowproduction of a beam of more suitable cross sectional shape.

Thus, it is seen that by the use of cylindrical lenses, focussing of thebeam with very high accelerating voltages and a large cathode focalaperture can be controlled with comparatively small magneto-motive forcein the individual lenses. The cylindrical lenses are more easilycorrected for aberrations. Thus, high beam intensities in a sharplydefined beam are obtainable in manner feasible with available materials.

This invention may be more easily understood by a reference to thespecification and to the accompanying drawing of which:

FIGURE 1 is a diagrammatic view of one embodiment of the apparatus inaccordance with this invention;

FIGURE 2 is a top view of a doublet cylindrical lens;

FIGURE 3 is a cross sectional view of the impinging beam or" chargedparticles;

FIGURE 3a is a curve of the intensity distribution along the X axis ofFIGURE 3 with intensity plotted along the axis of ordinates againstdistance plotted along the axis of abscissa;

FIGURE 3b shows the intensity distribution along the Y axis of FIGURE 3with intensity plotted along the axis of ordinates against distanceplotted along the axis of abscissa;

FIGURE 4 is a cross sectional view of the. beam of charged particlesproduced by two cylindrical lenses arranged with the primary axisperpendicular one to the ot er;

FIGURE 5 is a top view of a 4-fold cylindrical lens.

In the apparatus for producing and shaping a high intensity beam ofcharged particles for working material, shown in FIGURE 1, the beam isproduced by a system composed of a cathode 1, a control grid 2 and ananode 3. The cathode may be made of any conventional material but ispreferably a linearly developed cathode, that is, its extensionperpendicular to the plane of the drawing is greater than its extensionin the plane of the drawing.

The cylindrical lenses 4 and 5 for focussing the beam are arrangedbetween the anode 3 and the workpiece material 7 upon which the beam isto be focussed. A lens 6 is provided to generate a rotating field torotate the beam. The beam of charged particles 8 is focused by thecylindrical lenses 4 and 5 to have. its smallest cross sectional area inthe plane of the working piece 7.

The cylindrical lenses 4 and 5 are corrected with a respect toaberration by appropriate shaping of the pole shoes and/ or by mountingspecially shaped ferromagnetic members which influence the distibutionof the magnetic field in the air gap between the poles. Such correctivesteps are known to the art.

As can be seen more clearly from FIGURE 2, each of the cylindricallenses is comprised of 4-pole pieces 9, 18, 16 and 17 each of which isdeveloped as an equilateral hyperbole surrounded respectively byelectromagnetic coils 10, 15, 13 and 14. The magnetic field closesthrough the central aperture of the lens (through which the beam ofcharged particles passes) and through the ferromagnetic ring 11 joiningthe poles. The entire lens is contained within a container 12 ofnonmagnetic material. It has been found advisable to embed the polepieces and the coil of the lens in a potting compound 19 such assynthetic resin to prevent displacement of the pole pieces from theirdesired position during operation.

The cylindrical lenses are developed with the coils thereof lying in aplane perpendicular to the axis of a beam of charged particles. Thus, anextremely thin construction is obtained which is advantageous incorrecting the lens for aberrations and which ensures that the lensoccupies very little space in the vertical direction.

It is possible to use electro-static cylindrical lenses instead of theelectro magnetic cylindrical lenses shown in the drawing. However, theuse of electrostatic lenses makes it necessary to apply very highvoltages to the electrodes in order to focus particles of high velocity.

The beam focussed by only one cylindrical lens upon the surface of thematerial to be worked is shown in FIGURE 3. As is characteristic of acylindrical lens, the cross section 20 of the focus beam will extendalong a line. However, the intensity distribution within the crosssectional area, illustrated by curve 21 in FIGURE 3a and curve 22 inFIGURE 3b for the X and Y coordinates respectively is almost rectangularin shape. Thus the high intensity is substantially constant across thebeam and the beam peripheral edges are defined by a sharp drop inintensity.

In some applications, such as the drilling of round holes into a workpiece, the elliptical outline of the beam focussed upon the surfacemakes it diidcult to suitably control drilling by such beam. In suchapplications it is advantageous to employ a rotating field generated bylens 6 to rotate the focussed beam about its center of area. Rotation ofthe beam will produce a working beam having an effective cross sectionwhich is round, illustrated by broken line in FIGURE 3. Variation of thediameter thereof within predetermined limits can be eliectedconveniently by adjustment of the coil current in the cylindrical lens.

In some applications control of the rotating held in combination withthe control of the cylindrical lens is complicated. In such applicationsit is often preferable to employ two cylindrical lenses 4 and 5 as shownin FIG- URE l. The major axis of cylindrical lens 5 is perpendicular tothe axis of lens 4. Thus, the beam will be focussed successively by thetwo cylindrical lenses to give a cross section 23, FIGURE 4, of cloverleaf outline. It will be noted that additional cylindrical lenses may beemployed to shape the beam to more nearly circular cross sectionalshape. In such applications it is possible to dispense with the rotatingfield lens during working of the material including the drilling orround holes in the material.

It will be noted that the cylindricallenses 4 and 5 may be combined intoa single compound lens as is shown in FIGURE 5. In FIGURE 5 the lens 24comprises 8 poles with eight associated electromagnetic coils. Each ofthe pole pieces is developed as an equilateral hyperbole and theremaining structure is the same as set forth in the explanation of thecylindrical lens shown in Fl URE 2.

Thus, the apparatus in accordance with this invention allows thefocussing of a high intensity beam of charged particles having a sharplydefined cross sectional outline. The energy is derived from a chargeparticle emission system having a large focal aperture.

In welding materials by means of a beam or" charged particles a veryhigh intensity of the impinging beam is necessary. Therefore, it isespecially advantageous to employ the apparatus according to theinvention for such welding. This apparatus may further be employedadvantageously for drilling, milling, surface treating and hardening orother working of material by a beam of charged particles.

It will be noted that for some of these working efiects the beam may becontrolled by pulses during working of the material in manner known tothe art without deleteriously afiecting the focussing thereof.

It will be understood that this invention may be variously embodied andmodified the scope oi the subjoined claims.

I claim:

1. Apparatus for producing and shaping a high intensity beam of chargedparticles for working material comprising a beam producing systemconsisting of a cathode, a control grid, and an anode, said beamproducing system generating a beam of charged particles of large focalaperture, and an electromagnetic cylindrical lens posl tioned betweensaid anode and said material to focus said beam on said material in animpinging beam of high intensity with a sharply defined periphery atwhich the beam intensity decreases sharply, said cylindrical lens beingcorrected for aberrations.

2. The combination in accordance with claim 1 in which the beamproducing system comprises a linearly extending cathode aligned with themain axis of the cylindrical lens.

3. A combination in accordance with claim 1 which includes a secondcylindrical lens positioned adjacent said first lens with the main axisthereof perpendicular to the main axis of the first cylindrical lens.

4. A combination in accordance with claim 1 which includes a lens systemcomprising four pole pieces symmetrically spaced about an aperturethrough which the beam is directed.

5. A combination in accordance with clairnl which includes a lens systemcomprising eight pole pieces symmetrically spaced about an aperturethrough which the beam is directed.

6. A combination in accordance with claim 1 which includes means forproducing a rotating magnetic field across the axis of said beam andperpendicular thereto to rotate said beam about its axis.

7. A combination in accordance with claim 4in which said pole pieces aresurrounded by coils for establishing a magnetic field and in which saidcoils are positioned in a. plane perpendicular to the axis of the beamof charged particles.

3. A combination in accordance with claim 7 in which said pole piecesand coils of said lens are embedded in synthetic resin poured over them.

References Cited in the file of this patent UNITED STATES PATENTS2,sss,5s9 Kaiser et a1. Apr. 21, 1959 2,915,662 Giacchetti Dec. 1, I9592,919,381 Glaser Dec. 29, 1959 FOREIGN PATENTS 139,051; Great BritainOct. 26, 1955 whe

